SGML/XML major mode in files that do not declare a DOCTYPE

[Sam]

I split up large XML documents by placing large chunks into separate files, 
then declare them as entities:

<!ENTITY chapter1 SYSTEM "chapter1.xml">

My understanding is that chapter1.xml should not declare its own DOCTYPE. I 
have a single DOCTYPE in the main XML document, which includes all these 
entity definitions. However, when editing chapter1.xml, the XML and the SGML 
major modes do not work, since the file lacks its own DOCTYPE declaration.

What's the best way to get the XML or the SGML major mode working when 
editing chapter1.xml? I haven't had much luck Googling this.

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